The invention is concerned with resurfacing of brake disks, also known as rotors, and similar rotational articles having precision surfaces, such as clutch flywheels (with precision friction surface only on one side). More particularly, the invention relates to an apparatus and associated method for grinding the surfaces or these precision disks without surface imperfections or high spots which can be caused by "runout" of bearings.
Automotive brake rotors disks are conventionally resurfaced by use of lathe facing methods and apparatus, with the disk supported on ball or roller bearings. Although a relatively high degree of accuracy can be obtained using this conventional resurfacing procedure, there is at least a small degree of "runout" associated with all ball bearings or roller bearings, and this produces slight imperfections in the surfaced disk or rotor. The "runout" is due to a required clearance for movement of and with respect to the balls or rollers in the bearing, and can be further increased by eccentricity of members or wear or imperfection in the shape of the balls, rollers or other parts. The slight eccentricities or variations in the spinning of a disk supported by roller bearings or ball bearings causes undulations or high spots in the surface of the finished disk.
The result of these imperfectly formed disk finishes is a pulsation of the braking torque during application of braking in a vehicle. The driver and passengers feel this as an undulation or pulsed jerking of the vehicle upon brake application, and the driver often further feels this by a pulsation of the brake pedal itself.
The problem of brake torque variation, sometimes called "brake judder", is discussed in the SAE publication SP-914, section 3.2, "Brakes", pages 26-32. In that article it was recognized that disk thickness variations of six ten thousandths of an inch (0.0006") can cause discernible brake torque variation. However, this article and the industry in general did not recognize the cause of brake torque variation as relating so specifically to thickness variation on the disk or rotor caused by the "runout" of bearing supports during surfacing. The above article does state that if an installed "runout" is present on the disk face, wear during braking is concentrated at the high points, and the article indicates that most manufacturers have installed disk rotor "runouts" better than 0.080 mm. This reference to runout encompasses any eccentricity, play or warp in the rotor, typically 0.002 to 0.003 inch as manufactured, and does not refer to support bearings during surfacing. "Runout" in reference to disks or rotors, as opposed to bearings, refers generally to wobble of the entire disk relative to the shaft journal at the hub. This is different from disk thickness variation, which is more critical in producing undesired torque variation during braking. To some extent the brake pads and calipers can accommodate a certain amount of disk runout, without any brake pulsation or without noticeable brake pulsation. The article also notes that new disk brake rotors do have some initial disk thickness variation from the manufacturing process.
It has been found in accordance with the present invention that disk thickness variations even smaller than six ten thousandths of an inch can cause brake torque variation noticeable to the driver. Generally the thickness variation in a manufactured disk is 0.0006 inch or more, and in any event, the brake torque variation effect tends to increase during the life of the brakes.
Problems with disk thickness variation have been found to be exacerbated with anti-lock brake systems, which have sensors that sometimes tend to read a thickness variation on the disk surface, however slight, falsely as an imminent locking of a brake. This can tend to further increase the pulsation of the automobile and the brake pedal upon application of brakes, particularly heavy application of the brakes.
It is an object of the present invention to achieve near zero disk thickness variation in a surfacing or resurfacing process for brake disks or flywheels.